Optical arrangement of autofocus elements for use with immersion lithography

ABSTRACT

An autofocus unit is provided to an immersion lithography apparatus in which a fluid is disposed over a target surface of a workpiece and an image pattern is projected onto this target surface through the fluid. The autofocus unit has an optical element such as a projection lens disposed opposite and above the target surface. An autofocus light source is arranged to project a light beam obliquely at a specified angle such that this light beam passes through the fluid and is reflected by the target surface of the workpiece at a specified reflection position that is below the optical element. A receiver receives and analyzes the reflected light. Correction lenses may be disposed on the optical path of the light beam for correcting propagation of the light beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation of International Application No.PCT/US2004/011287 filed Apr. 12, 2004, which claims the benefit of U.S.Provisional Patent Application No. 60/464,392 filed Apr. 17, 2003. Thedisclosures of these applications are incorporated by reference hereinin their entireties.

BACKGROUND

This invention relates to an optical arrangement of autofocus elementsfor use with immersion lithography.

In semiconductor lithography systems in use today, automatic focusingand leveling (AF/AL) is typically accomplished by passing a low angle ofincidence optical beam onto the surface of a silicon wafer and detectingits properties after subsequent reflection from the wafer surface. Thewafer height is determined by optical and electrical processing of thereflected light beam. This beam passes under the last element of theprojection lens. The source and receiver optics are typically mounted toa stable part of the system, close to the projection optics mountingposition.

In immersion lithography, a liquid such as water fills the space betweenthe last surface of the projection lens and the wafer. At the edge ofthe water, typically at the edge of the lens or supported structure nearthe edge of the lens, the liquid-air boundary is not well defined and ischanging rapidly. It is not possible to transmit an AF/AL beam throughthis interface without substantial disruption and subsequent loss ofsignal, and hence performance.

It is therefore a general object of this invention to provide a way tointroduce AF/AL beams into the liquid layer without such disruption soas to preserve the optical accuracy and stability required.

More specifically, it is an object of this invention to provide anapparatus and a method for allowing AF/AL light beams to be used as inconventional lithography without the disrupting influence of the liquidimmersion boundary at the edge of the lens.

SUMMARY

Autofocus units according to this invention are for an immersionlithography apparatus that may be described generally as comprising areticle stage arranged to retain a reticle, a working stage arranged toretain a workpiece having a target surface, an optical system includingan illumination source and an optical element such as a lens positionedopposite and above the workpiece for projecting an image pattern of thereticle onto the workpiece by radiation from the illumination source,and a fluid-supplying device for providing a fluid into the spacedefined between the optical element and the workpiece such that thefluid contacts both the optical element and the target surface of theworkpiece. The optical element positioned opposite to the workpiece maybe treated as a component of the autofocus unit itself which may becharacterized as further comprising an autofocus light source serving toproject a light beam obliquely at a specified angle such that this lightbeam passes through the fluid and is reflected by the target surface ofthe workpiece at a specified reflection position that is below theoptical element, and a receiver for receiving and analyzing the lightbeam reflected by the target surface. Correction lenses preferably maybe disposed on the optical path of the light beam projected from theautofocus light source for correcting propagation of the light beam.

As an alternative embodiment, the optical element opposite the workpiecemay be cut on its two mutually opposite sides, and optically transparentwedge-shaped elements may be placed under these cuts such that the lightbeam from the autofocus light source will pass through them as it ispassed through the fluid to be reflected on the target surface of theworkpiece and to reach the receiver without passing through the opticalelement at all. In order to cope with the potential problem of bubblesthat may be formed due to the gap between the wedge element and theoptical element, the gap may be filled with a suitable material, madesufficiently narrow such as less than 2.0 mm such that capillary forceswill keep the gap filled with the fluid, or provided with means forsupplying a small suction to cause the fluid to move up through the gapor to supply the fluid such that the gap can be kept filled. Theboundary surface through which the light beam from the autofocus lightsource is refracted into the fluid from the interior of the wedgeelement need not be parallel to the target surface of the workpiece, butmay be appropriately sloped, depending on the indices of refraction ofthe materials that affect the design of the unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings of exemplary embodiments in which like reference numeralsdesignate like elements, and in which:

FIG. 1 is a schematic cross-sectional view of an immersion lithographyapparatus that incorporates the invention;

FIG. 2 is a process flow diagram illustrating an exemplary process bywhich semiconductor devices are fabricated using the apparatus shown inFIG. 1 according to the invention;

FIG. 3 is a flowchart of the wafer processing step shown in FIG. 2 inthe case of fabricating semiconductor devices according to theinvention;

FIG. 4 is a schematic side cross-sectional view of an autofocus unitembodying this invention;

FIG. 5 is a schematic side cross-sectional view of another autofocusunit embodying this invention;

FIG. 6 is an enlarged view of a circled portion 6 of FIG. 5; and

FIGS. 7, 8 and 9 are schematic side cross-sectional views of portions ofother autofocus units embodying this invention according to differentembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows the general structure of an immersion lithography apparatus100 that may incorporate the optical arrangement of autofocus elementsembodying this invention.

As shown in FIG. 1, the immersion lithography apparatus 100 comprises anilluminator optical unit 1 including a light source such as a KrFexcimer laser unit, an optical integrator (or homogenizer) and a lensand serving to emit pulsed ultraviolet light IL with wavelength 248 nmto be made incident to a pattern on a reticle R. The pattern on thereticle R is projected onto a wafer W coated with a photoresist at aspecified magnification (such as ¼ or ⅕) through a telecentric lightprojection unit PL. The pulsed light IL may alternatively be ArF excimerlaser light with wavelength 193 nm, F₂ laser light with wavelength 157nm or the i-line of a mercury lamp with wavelength 365 nm. In whatfollows, the coordinate system with X-, Y- and Z-axes as shown in FIG. 1is referenced to explain the directions in describing the structure andfunctions of the lithography apparatus 100. For the convenience ofdisclosure and description, the light projection unit PL is illustratedin FIG. 1 only by way of its last-stage optical element (such as a lens)4 disposed opposite to the wafer W and a cylindrical housing 3containing all others of its components.

The reticle R is supported on a reticle stage RST incorporating amechanism for moving the reticle R by some amount in the X-direction,the Y-direction and the rotary direction around the Z-axis. Thetwo-dimensional position and orientation of the reticle R on the reticlestage RST are detected by a laser interferometer (not shown) in realtime and the positioning of the reticle R is affected by a main controlunit 14 on the basis of the detection thus made.

The wafer W is set on a wafer holder (not shown) on a Z-stage 9 forcontrolling the focusing position (along the Z-axis) and the slopingangle of the wafer W. The Z-stage 9 is affixed to an XY-stage 10 adaptedto move in the XY-plane substantially parallel to the image-formingsurface of the light projection unit PL. The XY-stage 10 is set on abase 11. Thus, the Z-stage 9 serves to match the wafer surface with theimage surface of the light projection unit PL by adjusting the focusingposition (along the Z-axis) and the sloping angle of the wafer W by theauto-focusing and auto-leveling method, and the XY-stage 10 serves toadjust the position of the wafer W in the X-direction and theY-direction.

The two-dimensional position and orientation of the Z-stage 9 (and hencealso of the wafer W) are monitored in real time by another laserinterferometer 13 with reference to a mobile mirror 12 affixed to theZ-stage 9. Control data based on the results of this monitoring aretransmitted from the main control unit 14 to a stage-driving unit 15adapted to control the motions of the Z-stage 9 and the XY-stage 10according to the received control data. At the time of an exposure, theprojection light is made to sequentially move from one to another ofdifferent exposure positions on the wafer W according to the pattern onthe reticle R in a step-and-repeat routine.

The lithography apparatus 100 described with reference to FIG. 1 is animmersion lithography apparatus and is hence adapted to have a liquid(or the “immersion liquid”) 7 of a specified kind such as water fillingthe space between the surface of the wafer W and the lower surface ofthe last-stage optical element 4 of the light projection unit PL atleast while the pattern image of the reticle R is being copied onto thewafer W.

The last-stage optical element 4 of the light projection unit PL isaffixed to the cylindrical housing 3. In an optional embodiment, thelast-stage optical element 4 may be made removable for cleaning ormaintenance.

The liquid 7 is supplied from a liquid supply unit 5 that may comprise atank, a pressure pump and a temperature regulator (not individuallyshown) to the space above the wafer W under a temperature-regulatedcondition and is collected by a liquid recovery unit 6. The temperatureof the liquid 7 is regulated to be approximately the same as thetemperature inside the chamber in which the lithography apparatus 100itself is disposed. Numeral 21 indicates source nozzles through whichthe liquid 7 is supplied from the supply unit 5. Numeral 23 indicatesrecovery nozzles through which the liquid 7 is collected into therecovery unit 6. The structure described above with reference to FIG. 1is not intended to limit the scope of the immersion lithographyapparatus to which the methods and devices of the invention areapplicable. In other words, autofocus units of the invention may beincorporated into immersion lithography apparatus of many differentkinds. In particular, the numbers and arrangements of the source andrecovery nozzles 21 and 23 around the light projection unit PL may bedesigned in a variety of ways for establishing a smooth flow and quickrecovery of the immersion liquid 7.

FIG. 4 shows an autofocus unit 50 (not shown in FIG. 1) according tothis invention which may be incorporated into an immersion lithographysystem such as shown at 100 in FIG. 1, but the invention is not intendedto be limited by the specific type of the system into which it isincorporated. In this example, the last-stage optical element 4 of thelight projection unit PL is a hemispherically shaped projection lenswith its planar surface facing downward opposite to the upper surface(the “target surface”) of the wafer W, with a space left in between. Anautofocus light source 51 is arranged such that its AF/AL light beam 54,emitted obliquely with respect to the target surface of the wafer W,passes through a lower peripheral part of this lens 4 and then isrefracted into the immersion liquid 7 so as to be reflected by thetarget surface of the wafer W at a specified reflection position 55. Areceiver 52 for receiving and analyzing the reflected AF/AL light beam54 is appropriately positioned on the opposite side of the lightprojection unit PL. Numerals 53 each indicate what may be referred to asa correction lens disposed on the path of the AF/AL light beam 54 forcorrecting light propagation. Since the interface between the lens 4 andthe liquid 7 is well defined and essentially free of bubbles, the lightbeams are unimpeded and can provide good signals to maintain highaccuracy. In FIG. 4, broken lines indicate the exposure light cone, orthe boundary of the exposure region.

FIG. 5 shows another autofocus unit 60 according to another embodimentof the invention. Its components that are similar to those describedabove with reference to FIG. 5 are indicated by the same numerals. Theunit 60 shown in FIG. 5 is characterized as having the lower surface ofthe last-stage optical element 4 of the light projection unit PL cut intwo places facing respectively the autofocus light source 51 and thereceiver 52. The cut surfaces preferably may be flat, as shown in FIG.5, and the last-stage optical element 4 is still functionally andessentially a hemispherical lens. Optically transparent parts, referredto as wedge elements 61 and 62, are placed on both sides of the lens 4under these cut surfaces, the element 61 being on the side of theautofocus light source 51 and the element 62 being on the side of thereceiver 52. The cuts and the wedge elements 61 and 62 are designed sothat the AF/AL light beam 54 from the autofocus light source 51 willpass through the wedge element 61 and be refracted into the immersionliquid 7 without passing through the lens 4 and, after being reflectedby the target surface of the wafer W at the reflection position 55, willbe refracted into the wedge element 62 and received by the receiver 52again without passing through the lens 4. This embodiment isadvantageous because the wedge elements can be made of a differentmaterial from the lens element 4, such as optical glass.

The lower interface between the wedge elements 61 and 62 and the lens 4is important from the points of view of correct optical performance andgeneration of bubbles in the immersion liquid 7. With reference to FIG.6, which shows in more detail the portion of the wedge element 61 in aclose proximity of the lens 4, the gap D therebetween is a potentialsource of air bubbles, which may be entrained under the lens 4,adversely affecting its optical performance.

One of the solutions to this problem is to fill the gap with a suitablematerial or to press the wedge element 61 into contact with the lens 4such that the gap D becomes effectively zero and therefore does notperturb the liquid interface. Another solution is to keep Dapproximately equal to or less than 2.0 mm such that capillary forcescause the liquid 7 to fill the gap and keep it filled even while thewafer W is moved under the lens 4. A third solution is to supply a smallsuction to cause the liquid 7 to move up inside the gap D and to preventair from moving downward, as shown in FIG. 7 in which numeral 70indicates an air pump for providing the suction. FIG. 8 shows stillanother solution whereby a source 72 of the liquid 7 is supplied abovethe opening of the gap D to keep the gap D filled with the liquid 7.

The invention has been described above with reference to only a limitednumber of arrangements, but they are not intended to limit the scope ofthe invention. Many modifications and variations are possible within thescope of the invention. The shape of the wedge elements 61 and 62, forexample, need not be as described above with reference to FIG. 6.Depending, for example, upon the desired angle of incidence of the AF/ALlight beam 54 relative to the indices of refraction of the immersionliquid 7 and the material of the wedge element 61, it may beadvantageous, as shown in FIG. 9, to provide the wedge element 61 with asloped surface portion 64 such that the AF/AL light beam 54 passingthrough the wedge element 61 will be refracted into the immersion liquid7, not necessarily through a horizontal boundary surface as shown inFIG. 6, but through this appropriately sloped surface portion 64. Thiswill provide flexibility in the design of the arrangement embodying thisinvention.

FIG. 2 is referenced next to describe a process for fabricating asemiconductor device by using an immersion lithography apparatusincorporating a liquid jet and recovery system embodying this invention.In step 301 the device's function and performance characteristics aredesigned. Next, in step 302, a mask (reticle) having a pattern isdesigned according to the previous designing step, and in a parallelstep 303, a wafer is made from a silicon material. The mask patterndesigned in step 302 is exposed onto the wafer from step 303 in step 304by a photolithography system such as the systems described above. Instep 305 the semiconductor device is assembled (including the dicingprocess, bonding process and packaging process), then finally the deviceis inspected in step 306.

FIG. 3 illustrates a detailed flowchart example of the above-mentionedstep 304 in the case of fabricating semiconductor devices. In step 311(oxidation step), the wafer surface is oxidized. In step 312 (CVD step),an insulation film is formed on the wafer surface. In step 313(electrode formation step), electrodes are formed on the wafer by vapordeposition. In step 314 (ion implantation step), ions are implanted inthe wafer. The aforementioned steps 311-314 form the preprocessing stepsfor wafers during wafer processing, and selection is made at each stepaccording to processing requirements.

At each stage of wafer processing, when the above-mentionedpreprocessing steps have been completed, the following post-processingsteps are implemented. During post-processing, initially, in step 315(photoresist formation step), photoresist is applied to a wafer. Next,in step 316 (exposure step), the above-mentioned exposure device is usedto transfer the circuit pattern of a mask (reticle) onto a wafer. Then,in step 317 (developing step), the exposed wafer is developed, and instep 318 (etching step), parts other than residual photoresist (exposedmaterial surface) are removed by etching. In step 319 (photoresistremoval step), unnecessary photoresist remaining after etching isremoved. Multiple circuit patterns are formed by repetition of thesepreprocessing and post-processing steps.

While a lithography system of this invention has been described in termsof several preferred embodiments, there are alterations, permutations,and various substitute equivalents that fall within the scope of thisinvention. There are many alternative ways of implementing the methodsand apparatus of the invention.

1. An apparatus comprising: an optical element which projects an imageonto a first surface of a workpiece positioned adjacent to the opticalelement, there being a gap between the first surface of the workpieceand the optical element, the gap being substantially filled with animmersion liquid; an autofocus system including: an autofocus lightsource that emits a light beam through the immersion liquid onto thefirst surface of the workpiece; and a receiver that receives the lightbeam reflected off the first surface of the workpiece; and a transparentelement that causes the light beam to pass through an interface betweenthe transparent element and the immersion liquid, wherein a vacuum isprovided to a space between the transparent element and the opticalelement.
 2. The apparatus of claim 1, wherein the transparent elementcontrols the refraction of the light beam at the interface between thetransparent element and the immersion liquid.
 3. The apparatus of claim2, wherein the refraction of the light beam at the interface ranges from0 to 90 degrees.
 4. The apparatus of claim 1, wherein the transparentelement is made of a material selected from the group consisting ofglass, plastic, calcium fluoride and fused silica.
 5. The apparatus ofclaim 1, wherein the transparent element is of a shape selected from thegroup consisting of rectangles, squares, ovals, wedges and spheres. 6.The apparatus of claim 1, wherein the vacuum causes the immersion liquidto move up through the space between the transparent element and theoptical element so that outside air is prevented from moving into thespace.
 7. A method in immersion lithography, the method comprising:providing a workpiece having a first surface; positioning the firstsurface of the workpiece adjacent to an optical element; projecting animage through the optical element onto the first surface, there being agap between the first surface of the workpiece and the optical element;filling the gap substantially with an immersion liquid; passing anautofocus light beam from an autofocus light source through theimmersion liquid onto the first surface of the workpiece; and receivingthe light beam reflected off the first surface of the workpiece with areceiver, wherein the receiver receives the light beam through atransparent element that is separate from and positioned adjacent to theoptical element, and wherein a vacuum is provided to a space between theoptical element and the transparent element.
 8. The method of claim 7,wherein the transparent element controls the refraction of the lightbeam at an interface between the transparent element and the immersionliquid.
 9. The method of claim 8, wherein the refraction of the lightbeam at the interface ranges from 0 to 90 degrees.
 10. The method ofclaim 7, wherein the transparent element is made of a material selectedfrom the group consisting of glass, plastic, calcium fluoride and fusedsilica.
 11. The method of claim 7, wherein the transparent element is ofa shape selected from the group consisting of rectangles, squares,ovals, wedges and spheres.
 12. The method of claim 7, wherein the vacuumcauses the immersion liquid to move up through the space between thetransparent element and the optical element so that outside air isprevented from moving into the space.
 13. An immersion lithographyapparatus comprising: a reticle stage that retains a reticle; a workingstage that retains a workpiece having a first surface; an optical systemincluding an optical element, the first surface of the workpiece beingpositioned adjacent to the optical element to project an image onto thefirst surface, there being a gap between the first surface of theworkpiece and the optical element, the gap being substantially filledwith an immersion liquid; an autofocus system including an autofocuslight source that emits a light beam through the immersion liquid ontothe first surface of the workpiece and a receiver that receives thelight beam reflected off the first surface of the workpiece; and atransparent element that is separate from and positioned adjacent to theoptical element, wherein the receiver receives the light beam throughthe transparent element and wherein a vacuum is provided to a spacebetween the optical element and the transparent element.
 14. Theapparatus of claim 13, wherein the transparent element controls therefraction of the light beam at an interface between the transparentelement and the immersion liquid.
 15. The apparatus of claim 14, whereinthe refraction of the light beam at the interface ranges from 0 to 90degrees.
 16. The apparatus of claim 13, wherein the transparent elementis made of a material selected from the group consisting of glass,plastic, calcium fluoride and fused silica.
 17. The apparatus of claim13, wherein the transparent element is of a shape selected from thegroup consisting of rectangles, squares, ovals, wedges and spheres. 18.The apparatus of claim 13, wherein the vacuum causes the immersionliquid to move up through the space between the transparent element andthe optical element so that outside air is prevented from moving intothe space.
 19. A method for making an object using a lithographyprocess, wherein the lithography process utilizes the immersionlithography apparatus of claim
 13. 20. A method for patterning a waferusing a lithography process, wherein the lithography process utilizesthe immersion lithography apparatus of claim 13.